1. Field of the Invention
The present invention relates to an air conditioner, more particularly, to an apparatus for preventing liquid refrigerant accumulation of air conditioner and a method thereof.
2. Discussion of the Background Art
In general, an air handling unit is a device for air conditioning or heating a defined indoor space including a residential space, restaurant, or office. Recently, development for a multi air handling unit has been conducted for air conditioning or heating a multi-room indoor space more effectively by air conditioning or heating each room at the same time.
A cooling cycle is often applied to the air handling unit. In the cooling cycle, when working fluid passes through a compressor, an outdoor heat exchanger (condenser), an expansion valve, and an indoor heat exchanger (evaporator), it transfers heat from low-temperature refrigerant to high-temperature refrigerant to perform air conditioning or heating or air conditioning/heating.
FIG. 1 is a schematic diagram of a related art cooling cycle.
As shown in FIG. 1, the related art cooling cycle includes a compressing unit 2a, 2b with a plurality of compressors for compressing refrigerant to high-temperature, high-pressure vapor refrigerant; a plurality of oil separators 4a, 4b for separating oil from the refrigerant discharged from the compressing unit 2a, 2b; a four-way valve 6 disposed on the discharge side of the compressing unit 2a, 2b and changing the flow of the refrigerant in cooling mode or heating mode; an outdoor heat exchanger 8 for performing heat exchange on the refrigerant with outdoor air to condense the refrigerant to mid-temperature, high-pressure liquid refrigerant in cooling mode; an expansion valve 10 for passing the refrigerant (compressing the refrigerant in heating mode) that passed through the outdoor heat exchanger 8; an indoor heat exchanger 22 for performing heat exchange on low-temperature, low-pressure refrigerant that passed through an EEV 21 and the expansion valve 21 and indoor air; and an accumulator 12 for separating liquid refrigerant from the refrigerant that passed through the indoor heat exchanger 22 and supplying only vapor refrigerant to the compressing unit 2a, 2b. 
Here, the compressing unit 2a, 2b includes a constant-speed compressor 2a that rotates at a constant speed and an inverter compressor 2b whose rotational speed is variable and thus, air conditioning performance can be improved and air conditioning capacity can be varied as well. Inside of the constant-speed compressor 2a and inverter compressor 2b is oil to be circulated with the refrigerant, to improve reliability and working efficiency of the compressor.
Also, a first oil separator 4a is installed at the rear end of the constant-speed compressor 2a and a second oil separator 4b is installed at the rear end of the inverter compressor 2b, each separating oil from the refrigerant discharged from the respective compressors 2a, 2b and recirculating the refrigerant to each compressor 2a, 2b. 
In the case of a cooling cycle mounted with a plurality of compressors, it is important that the amount oil that circulates inside of each compressor should be same to operate the compressors more efficiently. To this end, a pressure regulating tube for equalizing pressure 14 is mounted in a lower end between the constant-speed compressor 2a and the inverter compressor 2b to maintain oil level inside the compressors 2a, 2b. 
When power is applied, the inverter compressor 2b immediately varies rpm (revolution per minute) in response to an input signal and adjusts refrigerant flow circulating in the cooling cycle to be adequate for cooling capacity. In the case that the inverter compressor 2b does not meet requirements for the cooling capacity, the constant-speed compressor 2a too adjusts refrigerant flow circulating in the cooling cycle to increase cooling efficiency.
Also, check valves 5a, 5b are disposed on the discharge side of the compressors 2a, 2b, respectively, to prevent the refrigerant from flowing inversely. That is, when only one of the compressors 2a, 2b is in operation, the check valve prevents the refrigerant from the condenser 8 flowing into the other compressor that is not in operation.
The accumulators 12a, 12b pass through only low-temperature, low-pressure vapor refrigerant generated by the indoor heat exchanger of an indoor unit 20 and if liquid refrigerant flows in, they accumulate the liquid refrigerant to prevent malfunction of the compressing unit 2a, 2b. 
In cooling mode, therefore, the compressed refrigerant by the compressing unit 2a, 2b of an outdoor unit 1 is radiated from the outdoor heat exchanger 8 and condensed, and then decompressed and expanded while passing through a corresponding EEV 21 of each indoor unit 20 through a service valve 16a. This decompressed, expanded refrigerant absorbs latent heat around the indoor heat exchanger 22 and is evaporated.
In heating mode, on the other hand, the compressed refrigerant by the compressing unit 2a, 2b is radiated from a corresponding indoor unit 1 and raises the indoor temperature, and then decompressed and expanded while passing through a corresponding EEV 21 of the outdoor unit 22 through the service valve 16a. This decompressed, expanded refrigerant absorbs latent heat around the outdoor heat exchanger 22 and is evaporated.
When the air conditioner is in standby mode, the compressors 2a, 2b and the four-way valve 6 are turned off.
In FIG. 1, the solid line indicates a refrigerant flowing direction when the air conditioner is in standby mode, and the slant line indicates a state where the four-way valve is off, and the refrigerant in the indoor unit and connecting pipe flows in the outdoor unit 1 and is accumulated in the accumulators 12a, 12b and partly in the lower part of the compressors 2a, 2b. 
At this time, if the refrigerant is accumulated in the lower part of the compressors 2a, 2b, it dilutes oil inside the compressors in operation and causes lubrication problems, resulting in damages on the compressors and decrease of reliability. In another case, it is also possible that the refrigerant accumulated in the accumulators 12a, 12b may over flow and flow in the compressors, bringing damages to the compressors 2a, 2b and decreasing the reliability of the compressors 2a, 2b. 
In the case of a multi-system, the longer the pipe is, the greater amount of refrigerant is distributed to the indoor unit 20, not the outdoor unit, and in the connecting pipe. Therefore, when the outdoor unit 1 is in standby mode, liquid refrigerant is accumulated even in the outdoor unit 1 more often.
FIG. 2 illustrates an oil recovery pipe structure in an accumulator.
As shown in FIG. 2, it is impossible to recover oil accumulated at the bottom of the oil recovery pipe and thus, the oil keeps accumulating at the bottom.
Due to the above problem, oil circulation amount is short and the reliability of the system is decreased.
FIG. 3 is a schematic diagram of a related art pipe structure around a compressor of an outdoor unit.
Pipes 152, 153 connected to a compressor 150 undergo a roofing treatment, and a separate focusing mass 140 is applied thereto. That is, low-temperature, low-pressure vapor refrigerant from an indoor unit (not shown) flows into the outdoor unit through an external pipe connected to a service valve 110, and liquid of this low-temperature, low-pressure vapor refrigerant is removed when it passes an accumulator 130 and the low-temperature, low-pressure vapor refrigerant is compressed by the compressor 150 and changes to high-temperature, high-pressure vapor refrigerant before flowing into a condenser.
Many times severe vibration occurs during the compressing process in the compressor 150 so it is necessary to control the transmission of this vibration to other parts of the system through suction and discharge pipes 152, 153 connected to the compressor 150. To control the transmission of the vibration, pipes can be elongated after the roofing treatment. Further, the focusing mass 140 made of elastic materials like rubber can be disposed at the lower portion of the roofed pipes, more particularly, at the lower end of roofing of the suction and discharge pipes 152, 153 of the compressor 150.
When the pipes connected to the compressor 150 and the accumulator 130 pass a reversing coil 120, the vibration is suppressed also. Here, the reversing coil 120 is disposed at an upper rear portion of the system not to interfere the pipes, and the entrance and exit of the reversing coil 120 are all faced to down.
Meanwhile, as for roofing of the suction pipe 152, the pipe is U bent from the accumulator 130 to the inverse direction and then L bent from the position of the reversing coil 119 upward and goes up straight. As for roofing of the discharge pipe 153, the pipe is U bent from a discharge unit to the inverse direction and U bent again along the bottom surface and then L bent from the position of the reversing coil 119 and lastly goes up straight.
Moreover, a vapor refrigerant pipe 151 for carrying the vapor refrigerant flowing in the compressor 150 does not undergo the roofing treatment but is connected directly to the reversing coil 120 and to the service valve 110 in consideration of the connection to an outside pipe.